1. Field of the Invention
The present invention relates to a method and apparatus for plasma processing a substrate, and more particularly to an improved method for determining an endpoint during a plasma etch process.
2. Discussion of the Background
Typically, during semiconductor processing, a (dry) plasma etch process is utilized to remove or etch material along fine lines or within vias or contacts patterned on a silicon substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a processing chamber. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure. Thereafter, a plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry.
Once the plasma is formed, selected surfaces of the substrate are etched by the plasma. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the selected regions of substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), low-k dielectric materials, poly-silicon, and silicon nitride. As the feature size shrinks and the number and complexity of the etch process steps used during integrated circuit (IC) fabrication escalate, the requirements for tight process control become more stringent.
Consequently, real time monitoring and control of such processes becomes increasingly important in the manufacture of semiconductor ICs. For example, one such monitoring and control diagnostic necessary for the timely completion of an etch step or process is endpoint detection. Endpoint detection refers to the control of an etch step and, in particular, to the detection of the feature etch completion or the instant in time when the etch front reaches the etch stop layer. If the etch process endpoint is improperly detected, then severe under-cutting of the feature may occur due to over-etching or partially complete features may result due to under-etching. As a result, poor endpoint detection could lead to devices of poor quality that are subject to increased risk of failure. Therefore, the accurate and precise completion of an etch process is an important area for concern during the manufacturing process.
One approach used for endpoint detection is to monitor the emission intensity of light at a pre-specified wavelength in time using optical emission spectroscopy (OES). Such a method might identify a wavelength corresponding to a chemical species present in the etch process that shows a pronounced transition at the etch process endpoint. Subsequently, a resultant signal is analyzed to detect distinct variations in the emission intensity, and the analysis of the resulting signal is then used to correlate with the completion of an etch process. Typically, the species selected corresponds to a reactive species or a volatile etch product. For example, the selected wavelength may correspond to CO* emission when etching SiO2 and polymer films, N2* or CN* emission when etching nitride films, SiF* emission when etching poly-silicon and AlCl* emission when etching aluminum.
In addition to the approach of monitoring the emission intensity at a single wavelength as described above, another approach is to monitor the light intensity at two wavelengths and record the ratio (or some mathematical manipulation thereof) of the two intensities. For instance, one wavelength is chosen for a specie whose concentration decays at an endpoint and a second wavelength is chosen for a specie whose concentration increases at the endpoint. Therefore, the ratio gives improved signal to noise.